How does the tyre pressure monitoring system work?
Tyre pressure monitoring systems (TPMS) keep an eye on each tyre’s air pressure and alert you if inflation falls below a safe level. There are two main approaches: direct TPMS, which uses sensors inside each wheel, and indirect TPMS, which infers pressure from wheel-speed data. When under-inflation is detected, a warning light usually appears on the dash, and some cars can display the current pressures.
Two core approaches to tyre pressure monitoring
These are the two primary methods used in modern vehicles to detect under-inflation and alert the driver.
- Direct TPMS (sensor in each tyre): Each wheel carries a sensor that measures absolute tyre pressure (and often temperature) and transmits the data to the vehicle’s TPMS controller. The system triggers a warning if any tyre drops below a predefined threshold. Many systems can also display the individual tyre pressures on the dashboard or infotainment screen. Sensors have built-in batteries with a typical lifespan of several years, and they are usually replaced or recalibrated when tyres are changed or if a fault is detected.
- Indirect TPMS (inference from wheel speeds): This method does not measure pressure directly. Instead, it uses wheel-speed sensors and ABS/ESC data to detect differences in the tyres’ circumferences that occur when a tyre is underinflated. When a discrepancy is detected, the warning light is triggered. Some systems can estimate approximate pressures, but they do not provide exact readings without direct sensors.
Direct TPMS provides precise readings for each tyre, while indirect TPMS relies on vehicle dynamics and speed data to flag potential under-inflation. Both aim to improve safety, fuel efficiency, and tyre wear.
How direct TPMS works
What it measures and how readings are used
In a direct TPMS, sensors mounted on each wheel (often at the valve stem or within the wheel assembly) continuously monitor the absolute pressure inside the tyre. The sensors transmit a unique ID and pressure value to the TPMS control module, which compares the results against the vehicle’s recommended cold inflation levels. If a tyre drops below the threshold, the system triggers a warning and may offer a live readout of each tyre’s pressure on the dashboard or infotainment screen.
Data transmission and processing
Sensor data is sent wirelessly to the vehicle’s TPMS receiver using radio frequency signals. Typical frequencies are around 315 MHz in North America and 433 MHz in many other regions. The TPMS control unit processes the incoming data, checks it against target pressures, and activates warning indicators when needed. Some vehicles also present real-time pressures for each tyre when requested by the driver.
Maintenance, calibration and replacement
Direct TPMS sensors are battery-powered and designed to last several years. When tyres are replaced or wheels swapped, a relearn or reinitialisation procedure may be required so the system recognizes the correct sensor IDs and positions. If a warning persists after inflating tyres to the recommended pressures, a faulty sensor or battery, or a leak, could be the cause and may require inspection by a professional.
How indirect TPMS works
Detection principle
Indirect TPMS uses the vehicle’s ABS wheel-speed sensors to monitor disparities in wheel rotation that arise when a tyre is underinflated. A tyre with lower pressure has a larger effective radius and may rotate at a different speed than correctly inflated tyres. The system analyzes these differences and flags under-inflation when the variance crosses a predetermined threshold. It does not display exact pressures on its own.
Limitations and applicability
Indirect TPMS can be cheaper and is common on older or more basic models. It may be less precise than direct TPMS and can be affected by tyre wear, mismatched tyres, or temperature changes. Recalibration or adjustment may be required after tyre service, rotation, or changes in tyre size.
What to do if TPMS warning lights appear
When the TPMS warning light comes on, follow these steps to ensure safe driving and accuracy of the system.
- Check all tyres with a gauge and inflate to the vehicle’s recommended cold pressures listed on the door jamb sticker or in the owner’s manual.
- Recheck the pressures after the tyres have cooled; adjust any tyre that is low to the specified level.
- If the light remains on after inflating, inspect for a puncture, leak, or faulty valve cap and ensure the sensor valve is secure.
- Consult the owner’s manual for the correct TPMS relearn/reinitialisation procedure if tyres were replaced, changed, or rotated. Some vehicles relearn automatically after driving a short distance, while others require a manual reset with a tool or through the infotainment system.
- If the light persists or you cannot locate a fault, seek diagnosis at a qualified tyre shop or dealership, as the issue could involve a faulty sensor or control module.
Addressing TPMS warnings promptly helps maintain safety, optimal fuel efficiency, and longer tyre life.
Maintenance, calibration and safety tips
Keeping TPMS accurate involves regular tyre checks and understanding relearn requirements after service. The following practices help ensure reliable operation.
- Keep all tyres inflated to the manufacturer’s recommended cold pressures
- Check tyre pressures when tyres are cold and before long trips
- If you replace tyres or wheels, have the TPMS relearn procedure performed as directed by the vehicle manufacturer
- Be aware that some direct sensors may require sensor replacement if damaged or the battery dies
- Remember that indirect TPMS relies on wheel-speed data and may need recalibration after tyre services
Regular checks and timely maintenance keep the TPMS accurate, supporting safer driving and better tyre performance.
Summary
Tyre pressure monitoring systems enhance safety by alerting drivers to under-inflation. They come in two main forms: direct TPMS, with sensors in each tyre reporting actual pressures, and indirect TPMS, which uses wheel-speed information to infer pressure differences. Understanding how each system works, common maintenance needs, and the steps to take when warnings appear helps drivers keep tyres properly inflated and reduce risk on the road.
